Method for making an optical filter for a character identification system

ABSTRACT

A source of collimated coherent light is arranged to impinge upon a character to be identified and a two-dimensional diffraction means is positioned in the path of the collimated light passing through the character to be identified, forming a plurality of spatially disposed diffraction patterns. Appropriate optical means of a determinable focal length is positioned one focal length from the position of the character to be identified, forming far-field images of the plurality of diffraction patterns. A filter has a plurality of spaced discrete areas each defining the distinctive diffraction pattern of a known character by holographic techniques. Optical means integrates the light passing through the filter, diverting it to a first order, offbeam position. A plurality of light-responsive means, disposed in the first order off-beam position, and each related to a discrete area of the optical filter produces a signal in response to a predetermined amplitude of light passing through the filter, therefore identifying the character which was initially exposed to the source of collimated coherent light.

United States Patent 151 3,656,838 Bryant et a1. [45] Apr. 18, 1972 [54]METHOD FOR MAKING AN OPTICAL FILTER FOR A CHARACTER PrimaryE.raminer-David Schonberg IDENTIFICATION SYSTEM AssistantExaminer-Robert L. Sherman Attorney-R. S. Sciascia, George J. Rubens andJ. W. [72] Inventors: John F. Bryant, Lemon Grove; Will T. M L

Hyde, Jr., San Diego, both of Calif. [73] Assignee: The United States ofAmerica as [57] ABSTRACT represented by the secretary of the Navy Asource of collimated coherent light is arranged to impinge [22] Filed;Sept 4 1970 upon a character to be identified and a two-dimensionaldiffraction means is positioned in the path of the collimated light 1 1PP 69,678 passing through the character to be identified, forming apluralit of s atiall dis osed diffraction atterns. Ap ro riate RelatedApphcamm Data opti al me ns of a deiierminable focal leiigth is positine d one [62] Division Of O 12, 1968, Pat. No. focal length from theposition ofthe character to be identified, ,571,603. forming far-fieldimages of the plurality of diffraction patterns. A filter has aplurality of spaced discrete areas each defining UsS. SF, thedistinctive diffraction pattern of a known character [51] h t. Cl...G02b 5/18 holographic technicluea Optical means integrates the light[58] Fleld of Search ..350/3.5, 162 SF; 250/219 CR; passing through thefilter, diverting it to a first Order Off-beam 356/71; 340/ 1463position. A plurality of light-responsive means, disposed in the firstorder off-beam position, and each related to a discrete [56] Referencescued area of the optical filter produces a signal in response to aUNITED STATES PATENTS predetermined amplitude of light passing throughthe filter, therefore identifying the character which was initiallyexposed Lohmann to the ource of collimated oherent 3,483,513 12/1969Burckhardt.. .....350/3.5 3,529,887 9/1970 Lu ..350/3.5 6 Claims, 11Drawing Figures PAIENTEDIPR 18 I572 SHEET NF 4 FILTER FOR FIG. 3

INVENTORS JOHN F. BRYANT BY W|1 T. HYDE,JR

SIGNAL 1, SIGNAL 2 ATTORNEYS mmmmmz 3,656,838

SHEET 2 BF 4 FIG. 4A

FIG. 40

INVENTORS JOHN E BRYANT BY Myra/Z55, QM QMW 4 TIOR/VE rs PATENTEUAPRI972 SHEET 4 CF 4 FIG. 56

FIG. 50

INVENTOR. JOHN F. BRYANT WILL 7: HYDE, JR

ATTORNEYS METHOD FOR MAKING AN OPTICAL FILTER FOR A CHARACTERIDENTIFICATION SYSTEM CROSS-REFERENCES TO RELATED APPLICATIONS Thisapplication is a division of application Ser. No. 774,737 now U.S. Pat.No. 3,571,603 filed Nov. 12, 1968.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may usedby or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

be manufactured and BACKGROUND OF THE INVENTION ployed principles of theFraunhofer diffraction phenomenon.

One such system, for example, employs a source of coherent light whichis directed to impinge upon a plane surface upon which the character tobe identified is defined. A translucent member is positioned fordisplaying the single Fraunhofer diffraction pattern which is developedand a plurality of optical means is arranged to generate a likeplurality of images of the single diffraction pattern. The plurality ofsuch images is then directed to a mask or filter having a plurality ofareas, each of which is distinguished by a different diffraction patternrepresentative of a particular distinctive character, symbol,alphanumeric, or arabic number, etc., it is desired to identify.

Other prior arts systems have employed the development of an opticalimage of the character or symbol to be identified to impinge upon alight-sensitive matrix which is scanned sequentially to develop matrixoutput signals characteristic of the image of the character or symboland which can be analyzed or compared with stored information toidentify the character or symbol represented by the matrix outputsignals.

All of these systems, however, develop a single diffraction patternwhich is either analyzed or caused by a purely optical lens-type meansto be reproduced for comparison with a mask or filter, matrix or otheroptically-sensitive means to identify the character represented by thediffraction pattern.

SUMMARY OF THE INVENTION The present invention contemplates a characteridentification system for rapidly and positively identifying characters,symbols, alphanumerics, arabic numbers, etc., which comprise a source ofcollimated coherent light, impinging upon a substantially plane surfacehaving relatively opaque and transparent portions defining the characterto be identified. A twodimensional diffraction means is positioned inthe path of the collimated coherent light which passes through therelatively opaque and transparent portions of the plane surface thusforming a plurality of spatially disposed diffraction patterns which aresubstantially identical and also distinctive of the character to beidentified. Optical means having a predetermined focal length ispositioned one focal length from the plane surface for forming afar-field image of the plurality of diffraction patterns. A spatialfilter is positioned one focal length from the optical means,substantially at the far-field image, the filter being divided into aplurality of spaced discrete areas, each area having relatively opaqueand transparent portions defining the distinctive diffraction pattern ofa different known character. Suitable optical means, such as a lens, ispositioned to integrate the light passing through the filter and directit to a first order, off-beam position. At the first order, off-beamposition, light responsive means is disposed at each of a like pluralityof discrete areas, each of which is related to a particular filter areafor producing a signal that corresponds to a predetermined amplitude oflight passing through the filter or mask. In this off-beam arrangement,the light having significant signal content is gathered and directed toactuate the appropriate light-responsive unit, while extraneous lightpasses along the principal optical axis avoiding undesirableinterference.

Moreover, the concept of the present invention inherently contemplates aunique method of making a filter or mask necessary for use in thesystem. The method includes the steps of intercepting a beam ofcollimated, coherent light with a character or symbol defined by therelatively opaque and transparent portions of a plane surface,generating diffraction patterns by positioning a two-dimensionaldiffraction means in the path of the light passing through the planesurface, focusing the diffraction patterns upon a recording plane, andsimultaneously illuminating the recording plane with collimated,coherent light generated by the same source as the beam and from anoff-axis position.

Thus, a characteristic diffraction pattern may be recorded upon thefilter or mask such as a photographic film or plate and such diffractionpatterns will contain not only amplitude information, but phaseinformation, as well, in accordance with holographic phenomenon as aresult of the secondary collimated beam of coherent light which impingesupon the recording plane of the filter or mask at the time thediffraction patterns are generated. The steps of creating a filter ormask in accordance with the method and concept of the present inventionmay employ a combination of equipment substantially the same as aportion of the equipment employed for the readout function previouslydescribed. In may include a source of a collimated, coherent beam oflight, a character or symbol usually depicted by contrasting opaque andtransparent portions in a determinable plane, a two-dimensionaldiffraction means, an appropriate lens or optical arrangement, arecording plane in the form of a photographic film or plate located onefocal length from the lens or optical arrangement, and a secondary,off-axis beam of collimated, coherent light developed from the samesource. The method of creating the filter or mask for each character orsymbol which is desired to be able to read out is sequentially processedin the aforegoing described manner so as to create distinctivediffraction patterns located at predetermined different spatial areas ofthe filter or mask.

OBJECTS OF THE INVENTION Accordingly, it is the primary object of thepresent invention to provide a character identification system ofimproved performance employing a two-dimensional diffraction means fordeveloping a multiplicity of spatially disposed diffraction patternswhich are distinctive of each character it is desired to identify.

Another most important object of the present invention is to providesuch a character identification system which obviates the need to employoptical means for developing a multiplicity of diffraction patternimages from a single diffraction pattern.

It is another most important object of the present invention to providean improved character identification system wherein a mask or filterincludes a plurality of spatially disposed diffraction patterns eachrepresentative of a different character and containing phase informationas well as amplitude information.

Another object of the present invention is to provide a characteridentification system wherein light sensitive identification means isdisposed in the first order off-axis beam of the optical arrangement toimprove its responsiveness and performance by minimizing extraneouslight interference.

A further object of the present invention is to provide such a systemwherein the diffraction means may be rotated about the principal axisrelative to the character or symbol to minimize overlap of the pluralityof diffraction patterns generated.

Another most important object of the present invention is to provide amethod for creating a mask or filter which includes a plurality ofdifferent diffraction patterns each distinctive of a different characterand containing phase information as well as amplitude information forimproved identification of each character.

These and other objects, advantages, and features of the presentinvention will be better appreciated from an understanding of theoperation of the several embodiments and methods disclosed hereintogether with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspectiveillustration of a preferred embodiment of the present invention;

FIG. 2 is a schematic, perspective illustration of the method of makinga mask or filter inherent in the concept and the teaching of the presentinvention;

FIG. 3 is an illustration of a typical filter or mask which may be madein accordance with the method taught by the present invention andemployed in the character identification system of the presentinvention.

FIGS. 4, A, B, and C are photographs illustrating diffraction patternsgenerated employing the concepts and teaching of the present invention.

FIGS. 5, A, B,C,D and E are photographs of larger, more detaileddiffraction patterns generated by different characters and employing theteaching and concept of the present inven- IIOII.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD FIG. 1 is aschematic, perspective representation of a system in the form of apreferred embodiment of the present invention. A source of collimated,coherent light is provided at and directed so that it impinges upon thecharacter depicted on the plate 11. The plane of the plate 11 mayinclude a photographic film or plate with relatively opaque andtransparent portions defining the character it is desired to identify.

A two-dimensional diffraction means 12 is placed to intercept thecollimated coherent light which passes through the character plane 11and may comprise a grating, grid, or screen appropriately configured toprovide two-dimensional diffraction. An optical means which may comprisea lens 13 having a determinable focal length is positioned at one focallength from the character plane 11 for forming far-field images in theform of a plurality of diffraction patterns which are distinctivelygenerated by the collimated, coherent beam passing through the characterplane 11 and the two-dimensional grid or diffraction means 12.Accordingly, there is generated a plurality of such substantiallyidentical diffraction patterns in a plane one focal length from theoptical means or lens 13. In the latter plane, there is positioned afilter or mask 14.

. The filter 14 is created and prepared in a manner which will bedescribed more fully hereinafter. It is sufficient for present purposesto point out that the filter 14 is divided into a plurality of spaced,discrete areas and each of the areas has relatively opaque andtransparent portions which define the peculiarly unique diffractionpattern associated with and distinctively related to a known character.Accordingly, when the plurality of diffraction patterns generated by thecharacter positioned in the plane 11 impinges upon the filter 14, onlyone of such patterns will match up in a congruent fashion and allowmaximum light to pass therethrough. A certain amount of light does passthrough, however, several of the other diffraction patterns associatedwith different characters.

Light passing through the filter 14 is collected by an optical means orlens 15 which has a determinable focal length and is positioned at onefocal length from the filter 14. The light collected by the lens 15 isdirected in part to a first order, off-axis beam position and focused ina plane one focal length from the lens 15. Positioned at the first orderoff-axis beam position and in the focal plane are a plurality oflight-responsive or photo-sensitive means, each of which is spatiallydisposed in that plane at a position which is related to, andcommensurate with, a discrete area of the filter 14 which defines aparticular diffraction pattern associated with a single character.

Accordingly, in operation, a character or a symbol such as the letter A"or any other distinctive character, symbol, alphanumeric, etc., ispositioned in the character plane 11 in the arrangement illustrated inFIG. 1 to intercept the collimated beam of coherent light which isproduced by the source 10. The collimated coherent light passes throughthe transparent portions of the plate or film depicting the character,impinging upon the two-dimensional diffraction means 12 which maycomprise a diffraction screen, grating or grid. A plurality ofsubstantially identical diffraction patterns are produced and arebrought into focus in a focal plane of the filter 14.

Filter 14, upon intercepting the diffraction pattern light, allows lightto pass through only those portions of the mask diffraction patternswhich are common with the diffraction pattern generated by thetwo-dimensional diffraction means 12 and are characteristic of thediffraction patterns generated by the particular character which ispositioned in the character plane 11. The light thus passing throughfilter l4is gathered by lens means or other appropriate opticalarrangement positioned in the plane of lens 15 and a single lightpattern is created by integration and directed to a position which iscorrelated with the character to be identified.

A plurality of light-responsive or photosensitive devices is positionedone focal length from the lens 15 at a first order, off-axis beamposition 16. Each of the photo-sensitive devices located in the plane 16is disposed in a position commensurate with, and related to, theposition of the diffraction pattern in the filter 14 which is uniquelydistinctive of a particular character. Accordingly, the light-responsiveor photo-sensitive means at each particular location is actuated toproduce a signal which positively identifies a correlated characterthrough the generation of uniquely distinctive diffraction patterns.

It should be noted that in the arrangement of FIG. 1, the use of atwo-dimensional diffraction means such as a screen, grating, or grid,inherent in the concept of the present invention, generates a pluralityof diffraction patterns rather than just a single diffraction pattern,therefore, obviating the need for employing relatively complex opticalmeans to develop a plurality of diffraction patterns for purposes offiltering and identification. Moreover, the filter 14 when produced bythe method conceived and taught by the present invention, contains notonly amplitude information, but phase information as well, employing theholographic phenomenon as will be more fully explained, hereinafter.

Additionally, in the arrangement as depicted in FIG. 1, thephotosensitive or light-responsive means positioned in the plane 16 isrendered significantly more sensitive and reliable to produce theproperly correlated output signals in that extraneous light which passesthrough the main axis of the system is prevented from reaching theplurality of photo-sensitive, light-responsive devices. This isaccomplished by having the photo-sensitive or light-responsive devicespositioned in the plane 16 in a laterally disposed relationship,substantially at the first order, off-axis beam position. Accordingly,the light gathered and integrated by the lens 15 and directed to thefirst order, ofi-axis beam position in the plane 16 contains a minimumof extraneous light which may otherwise interfere with the effectiveoperation and identification of each character as it passes through thecharacter plane 11 to be identified.

The concept of the present invention inherently requires a novel methodof developing and creating a filter or mask employing holographictechniques so that the filter contains not only amplitude informationbut phase information as well. FIG. 2 illustrates an arrangement ofequipment which may be used in carrying out the steps of the method tocreate the filter employed in the present invention. A collimated beamof coherent light is generated by source 20 much in the mannerillustrated in FIG. 1. That beam of collimated coherent light isintercepted by the character which it is desired to identify. Thecharacter may be the letter H as depicted in the character plane 21 ofFIG. 2 and may typically be a film, plate or other means to providerelatively opaque and transparent portions which cooperatively definethe character by allowing the collimated coherent light to pass throughthe transparent portions and by interrupting the passage of light at theopaque or nontransparent portions.

That amount of light which passes through the character plane 21impinges upon a two-dimensional diffraction means 22 which generatesreplication diffraction patterns uniquely typical of the characterinterposed in the character plane 21. The diffraction patterns arefocused by appropriate optical arrangement or lens 23 upon a discreteportion of a filter 24 located in a recording plane. The lens 23 ispositioned one focal length from the character plane 21 and also onefocal length from the recording plane which includes the filter 24. Thefilter 24 may be a photographic film or plate exposed for recording thediffraction pattern.

A collimated beam which may be generated by the same source as indicatedat 20 is directed obliquely as at 25 to impinge upon the filter 24concurrently with the diffraction pattern developed by thetwo-dimensional diffraction means 22. It should be noted that the angle0 between the obliquely oriented beam of collimated, coherent light, isthe same as the angle 6 illustrated in FIG. 2.

Thus, the light interference patterns which are established and recordedon the filter 24 contain phase information as well as amplitudeinformation in accordance with holographic techniques and phenomena.Each discrete area or section of the filter 24 is recorded in likemanner by uncovering a new area of unexposed film so that each newdiffraction pattern is directed to a different discrete area or sectionof the filter and a different filter area is uniquely associated witheach character. Accordingly, a multiple filter with any reasonablenumber of known diffraction pattern filter areas may be created asdescribed in connection with the explanation of the novel methodemploying the equipment of FIG. 2 and producing a multiplicity of uniquediscrete areas as illustrated by FIG. 3

FIG. 3 illustrates a multiple area filter which is divided into 49discrete areas in a 7X7 square configuration. As illustrated, thediffraction pattern filter for signal No. 1 containing the diffractionpattern information in accordance with the teaching of the methodpreviously described may be recorded in the first box at theupper-left-hand corner; the diffractions pattern for signal No. 2 may berecorded in the second box immediately adjacent that of signal No. 2,and the distinctive diffraction pattern for signal No. 3 in the thirdbox, etc., to form a number of n signal records in accordance with thecapacity of the multiple filter and the number of discrete areas intowhich it is divided.

FIG. 3 illustrates that the central discrete area is blacked out so asnot to be usable. The reason for this will be better understood byreferring to FIGS. 4, A, B, and C. FIG. 4A is a photograph of the actualreplication diffraction pattern produced by a system substantially ofthe type shown in FIG. 1 but without a character in the character plane,so that the replication diffraction pattern is substantially that of thebeam. It will be noted that a good multiple pattern is produced in a 7X7configuration in the middle of the entire pattern. However, thecentermost area within the 7X7 configuration contains a distinctlyhigher amplitude of light concentration as contrasted to the other areaswithin the 7X7 configuration. Accordingly, for reasons of practicaloperation and reliability of the system, it may be prudent to eliminatethe use of the central area by blocking it out as illustrated in FIG. 3.

FIG. 4B shows the multiple, distinctively unique, diffraction patternsgenerated in a system of the type illustrated in FIG. 1 by the insertionof the character H in the character plane ill. It will be seen, asillustrated in FIG. 48, that the character of the plurality ofdiffraction patterns produced is substantially identical, though varyingin size.

It may be found to be highly advantageous to rotate either the characteror the two-dimensional diffraction means about the axis of thecollimated beam to provide an improved separation of the plurality ofdiffraction patterns generated. FIG. 4C is a photograph illustrating themultiplicity of diffraction patterns generated by the character I-Iinserted in a system of the type illustrated in FIG. 1, with either thecharacter or the two-dimensional diffraction means rotated 45 relativeto each other and about the principal axis of the collimated beamgenerated by the source 20. With the improved separation realized bythis technique, there is less overlap of the signals as is clear bycomparison of the substantially same diffraction patterns generated bythe letter as shown in the FIG. 48 as contrasted to those of the FIG.4C.

FIGS. 5A through 5E illustrate larger diffraction patterns showing moredetail of the distinctively unique patterns developed by differentcharacters for purposes of demonstrating how different characters may bepositively and reliably identified employing the concept and teaching ofthe present invention.

FIG. 5A is a diffraction pattern generated by use of the presentinvention by inserting the letter A in a system of the type illustratedin FIG. ll.

FIG. 5B is a diffraction pattern generated by inserting the letter P ina system of the type illustrated in FIG. 1. It will be noted that whilethe letter A" is comprised entirely of straight lines and produces astraight line pattern, the letter P, being comprised of both straightlines and a curved portion, develops a diffraction pattern with bothstraight line and circular diffraction aspects.

FIG. 5C illustrates a diffraction pattern developed by the letter K wheninserted in the apparatus employed in the present invention as depictedin FIG. 1. It will be noted that the letter K, which is comprisedentirely of straight lines, produces a substantially straight linediffraction pattern, but at distinctly different angular aspects ascompared with the diffraction pattern produced by the letter A, therebymaking it uniquely distinctive and readily identifiable with the letterK 1 FIG. 5D is a photograph illustrating in considerable detail thedistinctively unique diffraction pattern generated by the letter O. Theletter O is entirely curvilinear in nature, and the diffraction patternsimilarly is entirely curvilinear in nature and devoid of any straightline characteristics.

This is in marked contrast to the diffraction pattern depicted by thephotograph of FIG. 5E (which is generated by the photograph of FIG. 5E)which is generated by the letter Q." Since the letter O used wascomprised of curvilinear as well as a small straight line portion, thediffraction pattern has both curvilinear and straight linecharacteristics.

It should be borne in mind that each of the diffraction patternsillustrated in FIGS. 5A through 5E represents only one of a plurality oflike patterns replicated in the use of the present invention.

Those knowledgeable in these arts will readily appreciate that theconcept of the present invention affords significantly improvedcharacter identification by including holographic information in boththe method of creating the mask or filter which is used in the system,and in the use of the novel system.

By combining data processing and computing techniques with the improvedsystem of the present invention, information contained in characters,symbols, etc., may be transformed or translated to enhance itsusefulness.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

We claim:

1. A method for making a filter for a character identification systemcomprising the steps of:

intercepting a beam of collimated, coherent light with a character orsymbol defined by relatively opaque and transparent portions of a planesurface,

generating a plurality of spatially disposed like diffraction patternreplications by positioning a two-dimensional difpositioning thetwo-dimensional diffraction means rotated about the principal axis ofoptical alignment relative to the character orientation.

4. The method of claim 1 wherein the collimated coherent lightilluminating the recording plane is derived from the same source as thebeam.

5. The method of claim 1 wherein the diffraction pattern image isrecorded photographically.

6. The method of claim 2 including the step of blacking out an axiallycentral discrete portion of a recording means.

1. A method for making a filter for a character identification systemcomprising the steps of: intercepting a beam of collimated, coherentlight with a character or symbol defined by relatively opaque andtransparent portions of a plane surface, generating a plurality ofspatially disposed like diffraction pattern replications by positioninga two-dimensional diffraction means in the path of the light passingthrough the plane surface, focusing the diffraction patterns upon arecording plane, and recording a selected diffraction pattern imagewhile illuminating the recording plane with collimated coherent light ofsubstantially the same character as the beam.
 2. The method of claim 1and further including the steps of sequentially recording a plurality ofdifferent characters, each on a discrete portion of a recording means.3. The method of claim 1 and further including the step of positioningthe two-dimensional diffraction means rotated about the principal axisof optical alignment relative to the character orientation.
 4. Themethod of claim 1 wherein the collimated coherent light illuminating therecording plane is derived from the same source as the beam.
 5. Themethod of claim 1 wherein the diffraction pattern image is recordedphotographically.
 6. The method of claim 2 including the step ofblacking out an axially central discrete portion of a recording means.